Redox

• What are reducing agents in redox reactions?

  Electron donors
• What are oxidising agents in redox reactions?

  Electron acceptors
• What does oxidation involve?

  Loss of electrons
• What is the oxidation reaction of zinc?

  Zn → Zn$^{2+}$ + 2e$^{-}$
• What does reduction involve?

  Gain of electrons
• What is the reduction reaction of chlorine?

  Cl$_{2}$ + 2e$^{-}$ → 2Cl$^{-}$
• What are the rules for assigning oxidation numbers?

  1. Uncombined elements = 0
  2. Sum in a compound = 0
  3. Monoatomic ion = ionic charge
  4. Polyatomic ion = sum equals charge
  5. Invariable oxidation numbers for certain elements
• What is the oxidation number of uncombined elements like Zn and Cl$_{2}$?


  Zero
• What is the oxidation number of Na in NaCl?

  +1
• What is the oxidation number of Cl in NaCl?

  • 1
• What is the oxidation number of Zn$^{2+}$?


  +2
• What is the oxidation number of Cl$^{-}$?


  • 1
• What is the oxidation number of carbon in CO$_{3}^{2-}$?


  +4
• What is the oxidation number of oxygen in CO$_{3}^{2-}$?


  • 2
• What is the oxidation number of Cl in CaCl$_{2}$?


  • 1
• What is the oxidation number of Fe in FeCl$_{3}$?


  +3
• What are the characteristics of redox equations and half equations?

  • Oxidation involves loss of electrons
  • Reduction involves gain of electrons
  • Oxidising agent is reduced
  • Reducing agent is oxidised
• What does the oxidising agent do in a redox reaction?

  Causes another element to oxidise
• What does the reducing agent do in a redox reaction?

  Causes another element to reduce
• Where are electrons placed in a reduction half equation?

  On the left
• Where are electrons placed in an oxidation half equation?

  On the right
• What are the steps to balance redox equations?

  1. Determine oxidation numbers
  2. Add electrons for change in oxidation number
  3. Check charge balance on both sides
• How do you balance half equations with varying amounts of oxygen?

  Add H$^{+}$, OH$^{-}$, and H$_{2}O$
• What is the half equation for MnO$_{4}^{-}$ to Mn$^{2+}$?


  MnO$_{4}^{-}$ + 8H$^{+}$ + 5e$^{-}$ → Mn$^{2+}$ + 4H$_{2}O$
• What is the first step in writing a half equation?

  Balance the change in oxidation number
• What is the final step in combining half equations?

  Add half equations and cancel electrons
• How do you combine half equations to form a full redox equation?

  • Ensure equal numbers of electrons
  • Add reduction and oxidation half equations
  • Cancel out electrons
• What is the half equation for SO$_{4}^{2-}$ to SO$_{2}$?


  SO$_{4}^{2-}$ + 4H$^{+}$ + 2e$^{-}$ → SO$_{2}$ + 2H$_{2}O$
• What is the oxidation number change of sulfur in SO$_{4}^{2-}$ to SO$_{2}$?


  +6 to +4
• What is the process for balancing more complex half-equations?

  1. Balance oxidation numbers with electrons
  2. Add H$_{2}O$ to balance oxygen
  3. Add H$^{+}$ to balance hydrogen
  4. Check charge balance